Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

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Re: Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

Post by Rhothgar »

Interesting. So if the rear suspension height is not set correct then this could affect rear braking power?

You can tell they don't get much power because the discs are designed so thin but it doesn't seem to feel right still.

You should surely be able to achieve lockout on the rear brakes with new discs and pads without additional load in the car?
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Re: Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

Post by Mandrake »

Rhothgar wrote:Interesting. So if the rear suspension height is not set correct then this could affect rear braking power?
Not really, as long as the ride height is in between the bump stops the pressure in the suspension is only dependent on the load weight. Consider that the rear ride height is changing constantly during driving as the height corrector attempts to make corrections for acceleration induced squat (and then undoes those changes a few seconds later) yet the brakes keep working normally.
You can tell they don't get much power because the discs are designed so thin but it doesn't seem to feel right still.

You should surely be able to achieve lockout on the rear brakes with new discs and pads without additional load in the car?
No, that's the whole point of the rear brake load compensation - to prevent the rear wheels from locking up when the car is unladden. Remember this design feature was introduced decades before ABS appeared, but was retained with the introduction of ABS, at least up until the Xantia. On a non-ABS model slamming on the brakes will lock the front wheels but seldom the rear wheels unless the tires are bald.

Locking the rear wheels (before the fronts) during braking is extremely dangerous as it can lead to uncontrollable over steer if it happens on a corner. Rear brakes are small by design as over 70% of the braking force is applied to the front brakes since most of the cars weight is thrown onto the front wheels during braking and the front of the car has more weight to begin with.

Rear brakes are there (on an unladden car) to apply enough braking to keep the rear pulled up straight, (so the back doesn't try to wander or fish tail) but not so much as to risk locking the wheels leading to over steer and loss of directional control. With trailing arm rear suspension the rear brake torque is also used to compress the rear suspension to minimise pitching under braking.
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Re: Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

Post by Rhothgar »

Thanks for the thorough explanation. However it doesn't address my concerns about the lack of real overall braking force.

All that changing the rear discs and pads has achieved is correction of the imbalance.
I cannot accept that they are only giving 100kgf a side. It is NOT enough.

MOT requires 65% of vehicle weight. Assuming 1350kg then that is 877.5kgf required (mental calculation).

Fronts were giving somewhere around 325-350kgf a piece so with 100kgf on each side at rear. So it is nominally in range. It is nowhere near good enough.

Citroens are renowned for having one if the best braking systems in the world.

Old BX's I have owned have probably performed at near 100% efficiency.
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Re: Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

Post by Hell Razor5543 »

It is a matter of a smart balancing act. The moment a wheel locks up the braking distance increases, and control decreases (especially if it is a steering wheel). Yes, ABS is designed to prevent the wheels from locking up, but the Citroen design has been around long before ABS was even thought of. Citroen have wisely kept this design, while complementing it with the ABS system. As the weight of the rear of the car directly affects how much braking force is felt on the rear brakes, so the rear suspension also should drop slightly (as fluid gets diverted from the suspension to the rear brakes), and this will help to keep the apparent weight roughly centralised, which should help keep all 4 wheels firmly in contact with the ground, improving overall braking.
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Re: Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

Post by Mandrake »

Hell Razor5543 wrote:As the weight of the rear of the car directly affects how much braking force is felt on the rear brakes, so the rear suspension also should drop slightly (as fluid gets diverted from the suspension to the rear brakes), and this will help to keep the apparent weight roughly centralised, which should help keep all 4 wheels firmly in contact with the ground, improving overall braking.
While you're right that the rear brakes prevent the rear suspension rising up under braking, its not because of fluid diversion. Press the brake pedal hard while the car is stationary - does the rear go down at all ? Not visibly, as the quantity of fluid removed is infinitesimal.

Its the action of the brake torque on the arm geometry. Imagine viewing the car side on from the right hand side. The rear wheel is rotating clockwise, you then apply the brake which tries to clamp the rotating wheel to the wheel hub imparting a clockwise torque on it proportional to the braking force.

This torque tries to rotate the trailing arm clockwise also, since the pivot point is at the body end of the arm the arm must rotate around this point and a clockwise rotation moves the wheel end of the arm up into the body - eg the ride height drops. Simple as that :)

If you can keep the rear height at or below the normal ride height during braking the amount the body weight throws forward dynamically is greatly reduced so braking is improved.
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Re: Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

Post by Stickyfinger »

citroenxm wrote:I always advise anyone to place TWO heavy concerete slabs in the boot.. you will get a good rear brake recording then...
or one of my girlfriends !
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Re: Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

Post by Rhothgar »

Surely you mean ex?
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Re: Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

Post by Stickyfinger »

Yep....a Jap whaling Ship got her off Portland Spit :)
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Post by addo »

No objection to BBW types from me! :twisted:
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Post by Stickyfinger »

addo wrote:No objection to BBW types from me! :twisted:
FlubberLover :)
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Post by addo »

Variety is the spice of life! :P Would you say no to a needy Nigella? She's not exactly thin these days...
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Re: Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

Post by Rhothgar »

Who would say no to Nigella?
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Re: Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

Post by Stickyfinger »

Nice VOLOVANTS!
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Re: Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

Post by harryp »

I was interested in the latter half of Mandrakes comment on the rear brakes just doing enough to keep the rear in line. Many years ago there was a trial carried out (yes, on the telly) with artics using a skid pan. To cut a long programme short; with no front brakes and with rears, at even gentle braking it immediately jacknifed. With the rears disconnected, under very heavy braking it pulled up in a straight line. IIRC they also tried braking on a curve - obviously not locking the fronts - and all was very stable. I was as surprised as the people conducting the test. Another point of interest (maybe) is that raising tyre pressures by a few psi delays the onset of aquaplaning by reducing the ability of the wedge of water to deform the leading edge and push completely under the tyre (I worked for a major tyre manufacturer, involved in the re-design of manufacturing and tyre test eqpt).
Used to have a Renault 20, and that had a brake force limiter on the rears. Many times had to convince MOT testers (before the days of the brake force rollers) to not leave the rear wheels dangling, or I invariably got the "rear brakes don't work mate". That was a very comfortable quiet car. I have also owned a couple of Renault 16's. The TL model was gutless but the TS was amazing once the valves had been radiused. It came to me from North of the border and was a "poor road model". With its fairly good ground clearance and vast (it seemed) suspension travel due to the all round torsion bar suspension, it was better than the Xant over speed humps, as at 40mph the travel absorbed them. A very odd experience, not carried out too often as I feared for the longevity of the structure!!!!
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Post by addo »

When you speak of disconnecting the artic's front brakes, do you mean the front axle pair, or the duals that do the driving?

16TS suspension isn't bad; reckon they had a good look at some parts of the Dee for ideas, even if it wound up quite different.
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